The present invention relates to air conditioning or environmental control systems and more particularly to a combined refrigerant heating and cooling system for vehicles.
Generally, automobiles, trucks and the like are ventilated by air forced into the interior by vehicle motion and by a variable speed, electrically driven blower. When heating of the vehicle interior is required, air is passed over a heater core. The core is connected to the engine water jacket and is an air-to-engine coolant heat exchanger. The interior or cabin temperature may be controlled by mixing or blending outside air with the heated air, mixing the heated air with recirculated air, or by a variation of the blower speed.
In order to cool the vehicle interior, a refrigeration air conditioning system is normally used. The system includes an evaporator core disposed in the interior of the vehicle, a refrigerant compressor driven by the vehicle engine, a condenser located in the engine compartment and an expansion valve. Refrigerant is pumped by the compressor through the condenser, expansion valve and the evaporator.
With combined air heating and air cooling systems, dampers and suitable ducts are used to control the proportion of fresh air and recirculated air to the heater core or the evaporator core. The air temperature is normally controlled by an interior thermostat which switches the compressor on and off through a magnetic clutch. Conventional vehicle air conditioning systems also include high and low pressure switches at the compressor discharge and suction to protect the system.
Recent generation engines, and particularly engines employed in trucks, have low flow coolant systems. Due to their low flow, insufficient heating capacity is available using a conventional forced air, heater core system, especially at low ambient temperatures. Conventional heaters suffer from time delay in achieving the desired heater output. Such heaters will not function until the coolant is heated. Coolant temperature varies as a result of different engine operating and vehicle loading conditions.
It has been proposed to use a heat pump for heating and cooling the vehicle interior. A heat pump is a refrigeration system which increases rather than decreases the amount of heat within the vehicle interior. A heat pump uses electrical or chemical energy to raise heat from a lower temperature to a useful level of 100.degree. F. or higher. The heat pump process differs from the more conventional refrigeration process only in that its purpose is to supply rather than extract heat from the interior of the vehicle.
A typical heat pump system includes a compressor, an outside heat exchanger or coil, an inside heat exchanger or coil and an expansion valve. When in the heating mode, the outside coil is an evaporator and ambient air is the heat source. The inside coil functions as a condenser and heats the air circulated over the coil. Conventional air-to-air heat pumps are of limited utility in a vehicle due to the wide range of ambient temperature conditions that may be experienced. At low ambient temperatures, insufficient heat capacity is available to maintain the vehicle interior at a comfortable temperature.
It has been proposed to combine heating and cooling in a refrigeration system for a vehicle wherein the engine coolant is used as the heat source in the heating mode. In such a system, refrigerant is passed by the compressor through an outside condenser and an inside coil which functions as an evaporator when air cooling is desired. When in the heating mode, the refrigerant is passed through a refrigerant-to-engine coolant heat exchanger which functions as an evaporator. The inside coil functions as a condenser. An example of one such system may be found in U.S. Pat. No. 2,801,827 entitled REFRIGERATING APPARATUS and issued on Aug. 6, 1957 to Dolza.
The system disclosed in the Dolza patent includes a refrigerant compressor driven by the vehicle engine through a V-belt and clutch. In the cooling mode, the compressor transfers compressed refrigerant to a condenser located in front of the vehicle, through an expansion valve and through an inside air-to-refrigerant heat exchanger. When in the heating mode, a reversing valve shuts off flow of refrigerant to the condenser. Compressed refrigerant flows through the inside heat exchanger which now serves as a refrigerant condenser. The refrigerant then flows through a line having a pressure regulating valve, an expansion valve, and a refrigerant-to-engine coolant heat exchanger. Coolant from the engine cooling system serves as a heat source in the heating mode. In order to increase the condenser pressure and hence increase the output capacity of the system when in the heating mode, the pressure regulating valve restricts the flow of refrigerant through the system. This results in an increase in the engine horsepower required to drive the compressor which should increase the rate of warm up of the car engine, thereby increasing the rate at which the engine coolant is heated.
Prior systems are not capable of relatively consistent heat output at the condenser side of the system from idling to full operating conditions. Prior approaches have been inefficient and are not capable of providing acceptable performance at the wide range of ambient conditions experienced. A need, therefore, exists for a system which can provide quick heat at engine start up, which is effective at extremely low ambient conditions, such as below minus 21.degree. F., and which permits effective control of the heating capacity of the system.